Shubo, the mother of sake. What is this essential stage in sake making?

As explained in a previous article, the three most important stages of the sake production process, in order of importance, are koji, moto, moromi. In this article, we take a look at the second. Moto is another way of referring to the Shubo. If you have ever studied the sake production process before, you will no doubt have come across this word. So without further ado, let’s look at this essential stage in more detail.

What is Shubo?

There are many translations for the shubo: starter, seed mash, mother-of-sake or mini-fermentation, and many more. It is in essence, a mini-fermentation. A smaller version of the main fermentation to help it get going.

The concrete definition of the purpose of the shubo is: “to mix together the steamed rice, koji and water to grow a healthy yeast”. That alone might not make much sense. To truly understand this stage requires a wider understanding of how all alcohol fermentation takes place. That is to say that, actually all alcoholic beverages, not just sake, are made from the fermentation of microorganisms. In the case of sake, yeast consumes glucose producing alcohol as a by-product through its anaerobic metabolism.
To put it a simpler way, without yeast there is no alcohol, or to be more precise, fermentation cannot take place if there is not a big enough population of yeast. The shubo is basically how brewers cultivate that population.

How is shubo made?

To make shubo, we first put koji and water into a tank called moto-oke and mix it together. Then we add a small amount of yeast and brewer’s lactic acid. Add to that some steamed rice and the base for shubo is complete. Brewers then let the mixture sit for two weeks to a month and to let the yeast multiply.

When making shubo the tank is left completely open. The door is left wide open. It’s an open invitation to outside bacteria and potentially destructive wild yeast to come in and enjoy the party; an invitation that can potentially destroy a batch of sake.

Thankfully, brewers can keep this bacteria out by altering the acidity level so that yeast which favours high acidity can survive while other bacteria which dislikes high acidity cannot. In modern times, brewers simply add lactic acid to do this.

Kimotokei and Sokujyoukei

The modern method of adding lactic acid is called sokujo (lit: fast method), so-called because all the bad bacteria is killed off early on allowing for brewers to quickly increase the temperature to facilitate sugar conversion and complete the process in a remarkable 14 days. The yeast has the right environment and enough nutrients to grow quickly and healthily. Sake made with sokujo is well-balanced with a consistent flavour.

Before brewers had the scientific knowledge to fully comprehend the microbial fight for survival that was going on inside the tank, they somehow managed to keep bacteria at bay through very risky trial and error.

One of the oldest, much riskier methods of shubo is called kimoto, and originates around the 1600s. Kimoto works by leveraging the lactic acid bacteria that form in the air around the tank during the early days of the process. Of course, these bacteria only form if the conditions are right. If you are a beer drinker, you might already be making comparisons with Lambic beer because it too is made through the exploitation of airborne bacteria. It takes one month for shubo to be made this way and requires much more hands because to entice the bacteria in, an ample supply of its nutrients, glucose is needed. That’s why you will see brewers hitting the mixture of rice and koji with wooden oars. This super tiring job is called motosuri or yamaoroshi (lit: knocking down the mountains / or heaps of mounded mixture) or as it’s often termed in English: oar-ramming. The oar-ramming helps to increase contact of the starch molecules with the koji enzymes which in turn speeds up the conversion of starch to glucose. It’s hard work but the result is a firmer flavour. Sake made with this method are labelled: kimototsukuri (made by kimoto method).

We hope that this short but simple introduction to shubo has helped shed some light on why this stage is so important and why after koji, this is the part of sake production that requires the highest level of skill. Understand shubo and your well on your way to understanding the sake making process in great depth. Next time you are at KURAND, see if you can spot a kimoto. Every now and then one makes its way into our lineup.